Method for creating a permeable fragmented zone within an oil shale formation



June-10, 1969 P. J. CLOSMANN ETAL 3,448,801

METHOD FOR CREATING A PERMEABLE FRAGMENTED zoun WITHIN AN on. same:FORMATION Filed July-l5, 1967 Sheet of 2 INVENTORS'. v

PHILIP J. VCLOSMANN HELMER ODE BY: THEIR ATTORNEY June 10, 1969 P. J.CLOSMANN ET AL 3,448,801

' METHOD FOR CREATING A PERMEABLE FRAGMENTED ZONE WITHIN AN OIL SHALEFORMATION Filed July 15, 1967 Sheet 2 of 2 I NVENTORS:

PHILIP J. CLOSMANN HELMER oo BY: m THEIR ATTORNEY United States Patent3,448,801 METHOD FOR CREATING A PERMEABLE FRAGMENTED ZONE WITHIN AN OILSHALE FORMATION Philip J. Closmann, Houston, and Helmer Ode, Bellaire,

Tex., assignors to Shell Oil Company, New York, N.Y., a corporation ofDelaware Filed July 13, 1967, Ser. No. 653,139 Int. Cl. E21h 43/25,43/24 US. Cl. 166247 9 Claims ABSTRACT OF THE DISCLOSURE A method forcreating a permeable zone in a subterranean oil shale formation byutilizing a plurality of strategically placed explosive devices ofvarying energy. A very high energy explosive, such as a nuclear device,is first exploded; then a plurality of explosive devices of lesserenergy, nuclear or non-nuclear, spaced radially from the high energyexplosive, are detonated between the time the initial cavity within theoil shale formation begins to expand radially outward as a result of thedetonation of the high energy explosive, and the time at which a chimneyof rubble of the fragmentsd oil shale formation is formed by thecollapsing of the cavity roof of the oil shale formation.

BACKGROUND OF THE INVENTION Field 0 the invention The present inventionrelates to a process for creating a zone of relatively high permeabilityin an oil shale; more particularly, it relates to a process of utilizinghigh and low energy explosives, strategically placed, to cause theformation of a fragmented rubble zone of the oil shale formationalfected by systematically detonating the explosives.

Description of the prior art The fact that thermonuclear explosives maybecome available for a fraction of a mill per kilowatt-hour equivalenthas led to the realization that ultra-high energy explosives can be usedin mining operations to break up formation and in the oil industry toincrease or stimulate productivity by heating or raising the pressure ofa reservoir.

One of the chief uncertainties with regard to the effects of nuclearexplosions within a subterranean oil shale formation is the permeabilitydistribution surrounding the cavity and subsequent chimney produced by adetonation. In prior tests, nuclear devices have been detonated withinvarious subterranean formations, and, at first, an almost sphericalcavity filled with hot gases was formed. This cavity expanded until thepressure within the cavity equaled that of the overburden. On cooling,the roof of the cavity collapsed, since it could not support itself anda so-called chimney developed within the formation. Chimney growthceased when the rock pile either filled the cavity or when a stable archdeveloped. Thus, the void space, or porosity of the broken rock in thechimney had approximately the same volume as the cavity before the roofbegan to fall in.

At the same time, the shock wave resulting from such nuclear explosionscreated a highly fractured region surrounding the chimney which maybecome many times larger in volume than the chimney itself. In an oilshale, the shale oil within the fragmented rubble zone including boththe chimney and the surrounding fractured region can be recovered byknown oil recovery means, such as in-situ retorting. One recoveryprocess would be to inject hot fluid into the zone so that the oil shalecomponents 3,448,801 Patented June 10, 1969 ice SUMMARY OF THE INVENTIONThe invention relates to a method for increasing the permeability in therigion immediately surrounding the primary rubble zone of an oil shaleformation. In a preferred embodiment, a primary nuclear device is placedwithin a subterranean oil shale formation and is surrounded by aplurality of radially-placed devices of lesser explosive energy, nuclearor non-nuclear. The radially- Iplaced devices are programmed to bedetonated by either the main shock wave from the primary device orexploded by other means after the main shock wave has passed. The lesserexplosive energy devices are preferably detonated between the time thespherical cavity caused by the explosion of the primary device begins toexpand radially outward and the time at which a chimney is formed by thecollapse of the cavity roof.

It is an object of this invention to increas the volume of a permeablezone of fragmented oil shale formation that is formed by detonating ahigh energy explosive device within the subterranean oil shaleformation.

It is a further object to create a rubble zone of relatively high anduniform permeability within an oil shale formation.

Further objects of the invention will become apparent as the followingdescription thereof proceeds in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 shows a verticalcross-sectional view of an oil shale formation prior to detonating aplurality of explosive devices within the formation;

FIGURE 2 is a diagrammatic view of the cavities formed by detonating theexplosive devices *within the oil shale formation of FIGURE 1 takenalong a plane through the lines 2-2 of FIGURE 1;

FIGURE 3 is a vertical cross-sectional view of the oil shale formationof FIGURE 1 after the primary explosive device has been detonated; and

FIGURE 4 is a vertical cross-sectional view of the final rubble zonecreated by detonating all of the explosive devices of FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGURE 1 shows subterranean oilshale formation 11 having a primary explosive device 12 located withinthe formation 11. Primary explosive device 12 is surrounded by aplurality of lesser energy explosive devices 13. The device 12 can beeither nuclear or non-nuclear; if a nuclear device is detonated in thesubterranean oil shale formation 11, a strong shock wave from thenuclear device begins to move radially outwardly, vaporizing, melting,crushing, cracking and displacing the oil shale formation 11. After theshock wave has passed, the high-pressure vaporized material expands, anda generally spherical cavity (i.e., the central cavity 14 in FIGURES 2and 3) is formed which continues to grow until the internal pressure isbalanced by the lithostatic pressure. The cavity 14 persists for avariable time depending on the composition of the oil shale formation 11and then collapses to form a chimney 15 (FIGURE 4). Collapse progressesupwardly until the volume initially in the cavity is distributed betweenthe fragments of the oil shale of formation 11. The size of thecylindrical rubble zone (i.e., the chimney formed by the collapse of thecavity 14 can be estimated from the fact that the initial cavity 14expands to the pressure exerted by the overlying portions of earthformations 11 and 16.

A zone of permeability 17 within the fragmented oil shale formation isformed surrounding the chimney 15 as can be seen in FIGURE 4. Thepermeability of this zone 17 can be increased by surrounding the primaryexplosive device which formed the central cavity with a plurality ofdevices 13 of lesser explosive energy. For example, in FIGURE 1, aprimary nuclear explosive device 12 is surrounded by explosive devices13, equally spaced from each other and radially spaced from the primaryexplosive device 12. These lesser energy devices :13 are preferably onsubstantially the same horizontal plane as the primary nuclear device(see FIGURE 1) and 500 to 1,000 feet from the nearest part of the outerwall of the central cavity 14 produced by the explosion of the highenergy nuclear device 12. The lesser energy devices 13 preferably havean energy yield no less than one-fourth of the primary high energy 12nuclear device and can be either nuclear or non-nuclear. Of course, thehigh energy device 12 could be non-nuclear and the lesser energy devices13 could be greater in number as long as the total of the individualenergy yield of the latter was approximately equal to the total combinedenergy yield of the primary explosive device. Their individual energyyields (i.e., the lesser energy devices 13) should be accordinglysubstantially equal to each other.

The lesser energy explosive devices 13 form cavities 18 (FIGURE 2) whendetonated, surrounded by fractured zones 19 as can be seen in FIGURE 2.The lesser energy devices 13 are preferably preset with detonating meansadjusted to explode upon arival of the main shock wave from theexplosion of the primary explosive device 12. Alternatively, the lesserenergy devices 13 can be suitably delayed to explode after passage ofthe main shock wave. Of course, another characteristic of the explosionof the primary explosive device 12 can be utilized to detonate thelesser energy devices 13, as, for example, changes in temperature orpressure as a result of the explosion of the primary explosive device.

Because of this time delay, either detonating the lesser energy devices13 upon arrival of the main shock wave or after the main shock wave haspassed but before the central cavity 14 becomes filled with rubble dueto the chimney collapse from above, the shock waves from the secondaryexplosions (that is, the explosions of the lesser energy devices 13)will cause spalling into the central cavity. The movement of rocktowards the central cavity 14 due to the satellite explosions willenhance the permeability in the regions between these explosions and thecentral cavity 14, by allowing development of a greater void space inthis region. This void space, indicated as a zone of increasedpermeability 17 in the drawings, has a high and uniform permeability inthe fragmented oil shale formation 11.

It can be seen from the foregoing that the lesser energy explosives 13are detonated substantially simultaneously with respect to each other sothat the energies from the pluralities of the explosions are additivelyfocused toward the cavity 14 formed by the central explosion of device.12. The detonations of the satellite or lesser energy explosives 13 areactuated in response to the arrival of the pressure Wave from thedetonation of centrally located nuclear explosive device 12 at alocation spaced generally horizontally from the central explosion. Thetime at which the lesser energy explosives 13 are detonated ispreferably between about the time the shock wave arrives at thesurrounding explosives and about the time the pressure in the centralcavity 14 becomes as low as the pressure of overburden. Stateddifierently, this time period lies between the period at which thespherical cavity 14 (FIGURE 3) formed by the central explosion begins toexpand radially outwardly and the time at which a chimney 15 is formedby the collapse of the cavity roof 20. The lesser energy explosives 13are thus spaced from the central cavity 14 by sufficient distances sothat the explosions from the satellite explosions are capable of causingrock to move into the void space within the central cavity 14 formed bythe higher energy explosive 12. The main shock wave from the higherenergy explosive 12, in the case of a nuclear device, travels at aboutfive meters per millisecond and the initial spherical cavity 14 persistsfor times that vary with the composition of the shale oil formation 11.Very little rock falls into this central cavity 14 until its pressurehas decreased to about the pressure of the overburden. The decrease inthe pressure of the central cavity 14 allows the weight of theoverburden to stress the arch of rock that forms the roof 20 of thecavity, and this initiates, or at least accelerates, the relativelyextensive collapsing that converts the spherical cavity 14 torubblefilled chimney 15.

In the prior tests, the radius of the fractured zone outside the centralcavity (the zone of increased permeability 17) was approximatelyproportional to the radius of the central cavity 14. This fractured zone17, created as disclosed in this application, has been found to have arelatively high and uniform permeability. Shale oil can be extractedfrom rubbled zones 15 and 17 by any known means, such as in situretorting. Suitable materials and techniques for use in treating thefragmented oil shale 11 within the permeable zones 15 and 17 aredisclosed in copen-ding applications, Ser. No. 632,006, filed Apr. 19,1967 and Ser. No. 656,815, filed July 28, 1967.

Various methods of carrying out the concepts of this invention maybecome apparent to one skilled in the art, and it is to be understoodthat such modifications fall within the spirit and scope of the appendedclaims- We claim as our invention:

1. A method of creating a zone of relatively high permeability within asubterranean oil shale formation com' prising the steps of:

placing a relatively high energy explosive device within the formation;

placing a plurality of devices of substantially lesser explosive energywithin the formation;

spacing the plurality of devices such a distance from the relativelyhigh energy device that the exploding of the plurality of devices cancause rubble from the oil shale formation to move into the area of thehigh energy explosive device;

exploding the relatively high energy explosive device Within the oilshale formation, thereby forming a cavity within the oil shale formationhaving a roof beneath the overburden which subsequently collapses toform a chimney of rubble within the oil shale formation; and explodingthe plurality of devices between the time the initial cavity within theoil shale formation begins to expand radially outwardly as a result ofthe explosion of the high energy explosive device and the time at whichthe chimney of rubble of fragmented oil shale formation is formed by thecollapsing of the cavity roof.

2. The method of claim 1 wherein the step of placing a relatively highenergy explosive device within the formation includes placing a nucleardevice within the formation.

3. The method of claim 1 wherein the step of exploding the plurality ofdevices includes the step of programming the plurality of devices to bedetonated as a result of a characteristic of the explosion of the highenergy explosive device.

4. The method of claim 1 wherein the placing of a plurality of devicesincludes placing a number of such devices whose energy yield aresubstantially equal to each other, the total energy yield of theplurality of devices 5 being substantially equal to the energy yield ofthe high energy device.

5. The method of claim 4 wherein the placing of the plurality of devicescomprises placing four such devices on the same horizontal plane as thehigh energy device and substantially equally spaced from the high energyexplosive device and from each other.

6. The method of claim 5 wherein the placing of the plurality of devicesincludes placing such devices approximately 500 to 1,000 feet from thenearest part of the outer wall of the cavity to he formed within the oilshale formation.

7. The method of claim 1 wherein the exploding of the plurality ofdevices includes exploding such devices after the main shock Wave fromthe high energy device has passed.

-8. The method of claim 1 wherein the exploding of the plurality ofdevices further includes exploding such de- References Cited UNITEDSTATES PATENTS 3,303,881 2/1967 Dixon 16636 3,342,257 9/1967 Jacobs eta1. 16636 FOREIGN PATENTS 1,147,517 11/1957 France.

1,278,435 10/ 1961 France.

STEPHEN I. NOVOSAD, Primary Examiner.

